The alternators are classified as under
(a) According to their field construction. As the field excitation of the alternators is concerned, these are classified as under.
(i) Stationary field type.
(ii) Rotating field type.
The alternators in which the field winding is kept stationary and armature rotating, are called stationary field type alternators. The alternators in which the armature is stationary and field is rotating are known as rotating field type alternator, as shown in Fig. 17.9. Generally rotating field type alternators are commonly employed.
Fig. 17.9. Stationary field type
(b) According to their pole constructions. These are classified as under:
(i) Salient pole type (ii) Smooth cylindrical pole type.
The alternators in which poles are projected having more diameter and less axial length are known as salient pole type alternators. The other which has got less diameter and more axial length and are Smooth, are called smooth cylindrical pole type alternators.
(c) According tot he number of phases. These are as follows:
(i) Single phase.
(ii) Three phase.
Generally poly phase alternators are employed.
(d) According to their field excitation. The alternators can be classified as follows:
(i) One, in which the field winding is excited from a separate source i.e. battery etc.
(ii) Other, in which the filed winding is excited from a D.C. generator mounted on the same shaft. The generator can be either a D.C. shunt generator or compound generator. A variable resistance is connected in series with the field winding to regulate the generated exciting voltage.